The present invention relates to a method and apparatus providing safer operation of Raman amplifiers (also known as distributed Raman amplifiers), especially in communication systems.
Distributed Raman amplifiers are become increasingly important features of today""s high bandwidth optical transmission systems. Raman amplification operates by sending a pump wavelength down the system fibre, producing additional amplification of the optical signal using the silica of the fibre as a gain medium.
In order for this to be effective, substantial pump powers have to be used. Typical pump powers are up to 1 Watt optical in the 1420-1480 nm band, and when combined with the signal, the laser power could in principle be as high as 1.5 Watts. The system is preferably safe in normal operation, since no light leaks from the fibre/cable/connector assemblies, However, when open to view, the light is Laser Class 4 (dangerous by direct and scattered viewing), and to conform to international standards, requires a closed and interlocked environment, specially trained personnel, and equipment such as laser safety goggles, for handling.
In a real communications system, however, this radiation could potentially be exposed in the open by cable break, or within a telecom hut by untrained personnel pulling connectors. The danger, particularly to eyes, is enhanced since the radiation is entirely invisible but is partially transmitted by the eye to the retina.
Although a great deal of work has been undertaken on the theory and application of Raman amplifiers, little has been done to implement appropriate safety systems to guard against accidental exposure to pump radiation in real communication systems.
Traditionally, laser protection in communication systems has been achieved in two ways, either individually or in combination.
Firstly, the system can look for a back reflection from the fibre that indicates a fibre break, or broken connection, and turn the optical signal amplifier(s) off when this back reflection is seen. This system is used for example in Ebrium Doped Fibre Amplifiers (EDFAs) to bring high output power down to a safe level in the event of a fibre break.
A second mechanism is to look for a loss of signal into the pre-amplifier in the system and use this as an indication of failure.
Both of these systems have significant limitations when used in conjunction with distributed Raman amplifiers.
In a Raman amplifier, the level of amplified spontaneous back-scatter in the fibre from a high level pump signal is too high to allow a cable break to be detected in this way. This spontaneous backscatter results from reflections from imperfect system components, Rayleigh scattering, and stimulated Brillouin scattering and cannot be avoided.
Looking for loss of signal power at the amplified wavelengths is potentially attractive, but in the absence of input signal a Raman amplifier can still generate significant amplified spontaneous emission at signal wavelengths. For example, a single wavelength in a functioning transmission system may have an optical signal to noise ratio (OSNR) of 20 dB in the bandwidth of the signal. However, the noise can occupy a bandwidth 300 times greater than the signal, so the ratio of total noise power to signal power at the output of a Raman amplifier can be less than unity. As a result, a reliable indication of loss of signal by direct power measurement cannot be made.
Embodiments of the present invention therefore aim to provide methods and apparatus enabling safer (improved) operation of Raman amplifiers, especially in optical communications systems, i.e. methods and apparatus which address and overcome, at least partially, one or more of the problems associated with the previous safety systems. Thus, embodiments of the present invention aim to provide methods and apparatus which offer improved protection, e.g. in communication systems, from accidental exposure to Raman amplifier pump lasers.
According to a first aspect of the present invention there is provided a method of operating Raman amplification pump lasers in an optical communication system, the method comprising the steps of:
pumping a length of optical fibre in a first direction with the output of a first Raman amplification pump laser, from a first end of the length of fibre towards a second end;
pumping the length of fibre in a second, opposite direction with the output of a second Raman amplification pump laser, from the second end towards the first end;
modulating the output of the second pump laser with a characteristic modulation;
detecting at the first end a predetermined parameter (i.e. characteristic or feature) of the characteristic modulation; and
reducing the pumping of the length of fibre in the first direction in response to ceasing to detect at the first end the predetermined parameter (i.e. in response to xe2x80x9closingxe2x80x9d the characteristic signal from the second pump laser).
The characteristic modulation may take a wide variety of forms. For example, it may be a digital modulation, such as a pulse pattern or profile superimposed on the laser output. Such a pattern can be recognised (detected) remotely using signal processing techniques.
Preferably, however, the modulation takes the form of modulating the output power with a characteristic frequency, and detection involves simply detecting a signal at the appropriate characteristic frequency, received from the xe2x80x9cotherxe2x80x9d pump laser.
Thus, as long as the signal from the second pump laser at the characteristic frequency (i.e. a unique identification frequency) is being received at the first end of the fibre, the fibre is known to be continuous (i.e. unbroken) from its first end to its second end.
More generally, as long as the parameter (i.e. characteristic) of the modulation applied to the second pump laser output can be detected (recognised) at the first end, the fibre link in between the two pumps is known to be intact. The method can therefore provide the advantage that the presence of the signal from the second pump laser, and therefore the continuity of the fibre, can be detected even when high levels of noise are being received at the first end as a result of back-scatter of the output from the first pump laser. Equally, when a break or some other kind of discontinuity in the fibre occurs, the loss of the signal at the characteristic frequency from the second pump laser can be detected at the first end even in the presence of strong reflection from the break of the output signal from the first pump laser.
Thus, the continuity of the length of optical fibre can be continuously monitored without the addition of significant hardware, complex control procedures, or specialised fibre cabling. The inventive method therefore provides a simple, cost effective way of safely operating Raman pump lasers in communication systems.
It will be appreciated that the terms xe2x80x9cfirst endxe2x80x9d and xe2x80x9csecond endxe2x80x9d are not intended necessarily to mean that the length of fibre has any discernible xe2x80x9cendsxe2x80x9d as such. These terms are simply used to denote positions along the length of fibre to assist in specifying the direction of pumping from the first and second lasers. In practice, the pump lasers will typically be coupled to the length of fibre using appropriately arranged fibre optic couplers.
When the parameter can no longer be detected at the first end (e.g. when the system detects a loss of the signal from the second pump laser at the first end), the pumping of the fibre in the first direction is reduced.
Reducing the pumping may involve a partial reduction in pumping power, or alternatively a full radiation to zero (or substantially zero), i.e. the pumping may be ceased. Preferably, the reduction may be achieved by reducing the power output of the pump laser, or by re-routing the power output. Preferably, when the pumping is arranged to cease in response to the loss of the signal or parameter detection this cessation simply involves the switching off/powering down of the first pump laser. This can be performed in less than one second, and so limits the time for which the first pump laser output can escape from any break or discontinuity in the fibre. The method therefore provides improved detection of fibre breaks, in response to which the first pump laser can be shut off to improve safety.
Preferably, the output power of the or each pump laser is modulated by applying a small additive modulated current to the laser bias current. Alternatively, an external modulation device (modulator) could be used, but this has the disadvantage of associated insertion loss.
The output power may, in other embodiments, be modulated in any characteristic, unique, detectable way, e.g. digitally.
The step of reducing the pumping of the length of fibre in the first direction may, advantageously, comprise the step of reducing the output power from the first pump laser, and this reduction may comprise the step of reducing the output power partially, or fully, i.e. to zero.
Advantageously, the method may further comprise the steps of modulating the output of the first pump laser with a different characteristic modulation (e.g. with a different characteristic frequency); detecting at the second end a parameter of the different modulation (e.g. by detecting a signal at the different characteristic frequency from the first pump laser); and reducing the pumping of the length of fibre in the second direction in response to ceasing to detect (i.e. no longer detecting) the parameter of the modulation (e.g. detecting a loss of the pump signal at the different characteristic frequency). This reduction may take any one of the forms described above, with reference to the pumping from the first pump laser. Thus, the method can provide a safe shut down of both pump lasers in response to a fibre break. Each pump laser has its output power modulated in a respective characteristic fashion. This modulation therefore provides a unique identification.
Preferably, the method may further comprise the steps of:
detecting at the second end an optical data signal conveyed by the fibre in the first direction; and
reducing the pumping of the fibre in the second direction in response to detecting a loss of said optical data signal.
Thus, the second pump laser may be shut down in response to a loss of a data signal resulting from a break in the fibre. When the second pump laser is switched off, the signal from it, with its characteristic modulation, is lost at the first end and accordingly the first pump laser can also be shut down. Thus, in a fraction of a second following a fibre break, both pump lasers can be safely powered down.
Advantageously, the step of detecting the optical data signal can comprise the steps of:
receiving at the second end a combined signal comprising the optical data signal superimposed on a background of noise;
using a periodic filter to split the combined signal into a first stream, comprising said optical data signal and noise, and a second stream, comprising said noise only;
monitoring the signal powers in each of the first and second streams from the periodic filter;
generating a difference signal indicative of the difference between the signal powers in the first and second streams, whereby said difference signal is indicative of the received optical data signal power; and
using said difference signal as an indicator of the presence or absence of said optical data signal.
This provides the advantage that the presence or otherwise of the optical data signal received at the second end can be detected even in the presence of broad band noise, and so offers an improved loss of data signal detection method and pump laser shut down compared with previous arrangements.
According to a second aspect of the present invention there is provided a method of operating Raman amplification pump laser in an optical communication system, the method comprising the steps of:
conveying an optical data signal along a length of optical fibre in a first direction, from a first end of the length of fibre towards a second end;
pumping the length of fibre in a second, opposite direction with the output of a first Raman amplification pump laser, from the second end towards the first end;
receiving at the second end a combined signal comprising the optical data signal superimposed on a background of noise;
using a periodic filter to split the combined signal into a first stream, comprising said optical data signal and noise, and a second stream, comprising said noise only;
monitoring the signal powers in each of the first and second streams from the periodic filter;
generating a difference signal indicative of the difference between the signal powers in the first and second streams, whereby said difference signal is indicative of the received optical data signal power;
using said difference signal as an indicator of the presence or absence of said optical data signal; and
reducing the pumping of the fibre in the second direction in response to detecting a loss of said optical data signal at the second end.
This provides the advantage that the presence and loss of the optical data signal can be detected even when noise levels are high (i.e. even in the presence of significant amounts of broad band noise) and so enables the pump laser to be powered down or shut down more reliably and quickly than in previous systems, thereby improving safety.
Preferably the method of the second aspect of the invention can further comprise the steps of pumping the length of fibre in the first direction with the output of a further Raman amplification pump laser, from the first end towards the second end;
modulating the output power of the first pump laser with a characteristic modulation;
detecting at said first end a predetermined parameter of the characteristic modulation; and
reducing the pumping of the length of fibre in the first direction in response to ceasing to detect (i.e. no longer being able to detect) the predetermined parameter at the first end.
This provides the advantage that as soon as the loss of the optical data signal is detected at the second end, the laser pumping the fibre in the second direction can be shut down, which then results in shut down of the laser pumping the fibre in the first direction. Thus, loss of the data signal at the second end triggers safe shut down of the laser pumps pumping the fibre in both directions, thereby significantly improving safety.
According to an embodiment of a third aspect of the present invention there is provided an amplifier unit for an optical communication system, the amplifier unit comprising:
a Raman amplification pump laser adapted to pump a length of optical fibre with output power;
a detector adapted to detect a signal at a characteristic frequency received by the amplifier unit via the optical fibre from another Raman amplification pump laser pumping the length of fibre with output power modulated at the characteristic frequency; and
a controller arranged to reduce the output power of the pump laser in response to the detector detecting a loss of the signal at the characteristic frequency.
Rather than using a characteristic frequency, the output of the other Raman pump can be modulated in some other characteristic way, and the detector may be arranged accordingly, for example to detect just a single characteristic parameter of the characteristic modulation.
Preferably, the output power of the Raman amplification pump laser is also modulated, at a different characteristic frequency.
According to a fourth aspect of the present invention there is provided an amplifier unit for an optical communication system, the amplifier unit comprising:
a Raman amplification pump laser adapted to pump a length of optical fibre;
a periodic filter adapted to receive a combined signal comprising an optical data signal and noise from the optical fibre, and to split the received combined signal into a first stream comprising the optical data signal and noise, and a second stream comprising the noise only;
a first photodetector arranged to generate a first power signal indicative of the signal power in the first stream;
a second photodetector arranged to generate a second power signal indicative of the signal power in the second stream;
a difference signal generator arranged to generate a difference signal indicative of a difference between said first and second power signals, said difference signal being indicative of the received optical data signal power; and
a controller arranged to reduce the output power of said pump laser in response to said difference signal.
The period filter used in embodiments of the present invention may take a number of forms, and may be made in a number of ways. Ways of making suitable periodic filters include e.g. using unbalanced Mach-Zhender interferometer, array waveguides, polarisation (birefringence) filters, dielectric filters, etc. These are available from a number of manufacturers like JDS, ITF Optical, Avanex, Chorum, and Bookham. These methods, and these types of periodic filters are well known.
In an amplifier unit in accordance with the fourth aspect of the present invention the pump laser may be adapted to provide output power (i.e. power for pumping the fibre) which is modulated in a characteristic way (e.g. modulated at a characteristic frequency).
Preferably, the unit may further comprise a pump signal detector for detecting a pump signal at a different characteristic frequency received by the amplifier unit via the optical fibre from another Raman amplification pump laser pumping the length of fibre with modulated output power, and
the controller may be further arranged to reduce the output power of the pump laser in response to the detector detecting a loss of the pump signal at the different characteristic frequency.
The pump signal detector may, alternatively, be arranged to detect other parameters of the characteristic modulation.
A further aspect of the present invention provides a communication system comprising a plurality of amplification units linking a chain of lengths of optical fibre, each amplifier unit comprising a Raman amplification pump laser arranged to pump an adjacent length of fibre with output power modulated with a respective characteristic modulation, each amplifier unit being arranged to detect a predetermined parameter of a different characteristic modulation applied to the output power of an adjacent amplification unit, each amplifier unit being arranged to reduce the output power of its own pump laser in response to ceasing to detect the predetermined parameter of the characteristic modulation of an adjacent amplification unit.
Preferably, each unit""s pump laser output is modulated at a respective characteristic frequency, and each unit is arranged to detect a signal from adjacent units at their respective characteristic frequencies.
Preferably, the or each pump laser output power is modulated by applying an additive modulated current to the pump laser bias current.
Alternatively, a modulator (i.e. external device) can be used to modulate the pump laser output, but this involves associated insertion loss.
The term xe2x80x9cRaman amplification pump laserxe2x80x9d is intended to encompass a variety of known lasers, including fibre lasers. Fibre lasers may be used as Raman pump lasers in certain applications because they give much higher output powers. A fibre laser is essentially a short wavelength laser that is converted down in frequency using the Raman effect. Fibre lasers can be modulated by modulation of the pump drive current in the same way.
It will be appreciated that the terms xe2x80x9creducing the pumpingxe2x80x9d and xe2x80x9creducing the output powerxe2x80x9d used throughout this specification are intended to cover both partial reductions and full reductions (i.e. reductions to zero) unless expressly stated otherwise.